The 16 connection cryostats for the Large Hadron Collider (LHC) being built at CERN are designed to fill the gap existing between the dispersion suppressor zones and the standard arcs of the accelerator. The first connection cryostat was cold tested down to superfluid helium temperature in August 2005, and the measured thermal performance was as expected. This paper presents the test results and a new thermal modeling of the connection cryostat based on the measurement of the thermal resistances of the braids used for thermalisation, allowing the precise determination of cool down times and equilibrium temperatures of the shielding under various conditions such as lead heating

Cano-Fernandez, E

Colombet, T

Marque, S

Poncet, A

Sitko, M

CERN Document Server

CERN, Accelerator Technology Department, Geneva, SwitzerlandEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsTHERMAL PERFORMANCE OF THE LHC CONNECTION CRYOSTATT. Colombet , E. Cano Fernandez, S. Marque, A. Poncet, M. SitkoThe 16 connection cryostats for the Large Hadron Collider (LHC) being built at CERN are designed tofill the gap existing between the dispersion suppressor zones and the standard arcs of the accelerator.The first connection cryostat was cold tested down to superfluid helium temperature in August 2005, andthe measured thermal performance was as expected. This paper presents the test results and a newthermal modeling of the connection cryostat based on the measurement of the thermal resistances of thebraids used for thermalisation, allowing the precise determination of cool down times and equilibriumtemperatures of the shielding under various conditions such as lead heating.Presented at the Cryoprague 2006 Conference17-21 July 2006 - Praha, Czech RepublicGeneva, Large Hadron Collider ProjectCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 993Abstract14 February 2007   1   THERMAL PERFORMANCE OF THE LHC CONNECTION CRYOSTAT  Colombet T., Cano E. F., Marque S., Poncet A., Sitko M.  AT Department, CERN, 1211 Geneva 23, Switzerland   The 16 connection cryostats for the Large Hadron Collider (LHC) be-ing built at CERN are designed to fill the gap existing between the dispersion suppressor zones and the standard arcs of the accelerator. The first connection cryostat was cold tested down to superfluid he-lium temperature in August 2005, and the measured thermal perform-ance was as expected. This paper presents the test results and a new thermal modeling of the connection cryostat based on the measure-ment of the thermal resistances of the braids used for thermalisation, allowing the precise determination of cool down times and equilib-rium temperatures of the shielding under various conditions such as lead heating.    THE CONNECTION CRYOSTATS (CC)  These cryogenic assemblies of 13 m length house a pseudo cold mass comprising the set of cryogenic, electrical busbars and vacuum conduits which ensure the continuity of the LHC main cryostat utilities (see Figure 1). Two pairs of main quadrupole busbars and one pair of main dipole busbars transit respectively through the M1, M2 and M3 tubes, connected at each CC extremities to the main magnet cold masses, filled with pressurized superfluid helium at 1.9 K (see Figure 2). The V1 and V2 beam pipes are cooled via an annular bath of pressurized superfluid helium contained in coaxial tubes. This annular space has direct links with the M1 and M2 lines via flexible hoses. Half way through the cold mass, the M and X lines are con-nected to a central helium reservoir, the so-called shuffling module, which houses the busbar loops (lyras) used to compensate for thermal contraction between room temperature (RT) and 1.9 K. In order to protect from beam-induced radiation the sensitive components and appara-tus installed around these cryostats in the LHC tunnel, lead shielding plates are installed around the beam pipes inside the cold mass.             Figures 1&2.  Completed connection cryostat and its cold mass M1XV1M2V2M3   2MECHANICAL AND THERMAL DESIGN  As shown on Figure 3, the CC structure is of a “cage” type. The cold mass is composed of 10 identical segments of ~1.2 m length, each housing a ~170 kg lead box composed of plates enclosing the V1 and V2 beam tubes. The structure is thermalized via 4 OFHC copper braids (transverse section of 4*400 mm2) clamped to the M1 and M2 heat sinks. The lead shielding is thermalized by its welded supporting structure, and by a copper link equipped with stainless steel extremities welded on the holding lead bracket and the M1 line.             Figure 3.  Thermalization of a cell of the cold mass. Position of the temperature sensors    Although the central shuffling module includes a heat exchanger with the X line, provid-ing a few watts of cooling power at 1.9 K, the main source of heat extraction is conduction in superfluid helium through the M lines to the adjacent cryomagnets. The transverse area of the superfluid helium conduits (M, N lines, and annular space around V1 and V2) have been sized to provide a constant temperature of 1.9 K (within 0.1 K) for the maximum specified heat loads of 0.95 W/m, of which 0.2 W/m are deposited in each of the V1 and V2 beam lines. The cool down of the whole structure from RT to a stable operating temperature must take less than 6 days [1]. Between RT and 4.5 K the cool-down is ensured by a forced flow of gaseous helium through the M lines (100 g/s). During cool down, temperature gradients across the mechanical structure must remain within allowable limits [2].    THERMAL PERFORMANCE  A CC equipped with 40 calibrated carbon-type temperature sensors, placed in strategic loca-tions of the cold mass (see Figure 3), was connected to an LHC magnet cold test feed-box providing helium gas for the cool down, liquid helium to fill the tubes of the cold mass, and a pumping line connected to the X tube heat exchanger.            Figure 4.  Schematic of the test-set-up, with the CC connected to CFB (Cooling Feed Box) Cold mass shell Lead box Copper braid Copper bracket Stainless steel end fitting Lead bracket Copper braid    3 Table 1 gives a summary of the values obtained for the cooling rates in various parts of the cold mass and the equilibrium temperatures achieved, in line with the expectations, despite the limited cooling power available and parasitic heat in-leaks of the test stand [3].  Table 1.  A summary of cool down rates and final equilibrium temperatures, expected and achieved   Cool down Cool down Final state  300K to  90K after 33 hours From 90K to  4K  (136  hours) at 1.9K  (42 hours)  Tempera-ture End state expected Tempera-ture expected Temperature M tubes 115 K stabilised  4.1 K 4K 1.9 K Cold mass up 240 K 1.83 K/hour 2.11 K/hour 9.8 K 7.1 K 6.9 K Cold mass down 225 K 1.83 K/hour 2.11 K/hour 9.1 K 5.3 K 5.5 K Copper braid hot clamps 200 K 2.63 K/hour  4.3 K  2.2 K Copper braid cold bracket 150 K stabilised  4.6 K  4.5 K Lead block 230 K 2.18 K/hour  5.8 K 5 K - 7 K 4.5 K  The efficiency of the copper thermalization braids was confirmed. The final state equilibrium temperatures were as expected. However the fact that the lead shielding blocks could reach temperatures at which it would be superconducting (lead critical temperature, no field, of 7.2 K), was identified as a potential problem to the LHC beams in case of  resistive transition [5].   A SIMPLIFIED THERMAL SIMULATION MODEL To determine precisely the equilibrium temperatures of the lead shielding and therefore assess  the real risk of having the lead superconducting in nominal machine conditions, a semi-empirical thermal model based on the measured thermal resistances linking together the cold mass shell, the lead shielding, and the lines M (cold sink) was used [4]. This model, based on the schematic shown in Figure 5, solves the time dependent heat transfer equations of a seg-ment of CC containing one lead shielding block, for the two positions relevant to the thermal structure for which temperatures were recorded:   ( ) ( ) 011 1211313=−−+−+ PTTRTTRdtdH L    ( ) ( ) 011 2211322=−−−−+ PTTRTTRdtdH S     Figure 5.  Schematic of a three thermal resistance model of the CC and governing equations   HL and HS are respectively the lead block and shell segment enthalpies at temperatures T1 (lead) and T2 (shell) during cool down. T3 is the temperature of the heat sink (M lines), R2 and R3 are the thermal resistances of the two types of copper braids used respectively to thermalize R3(T, t)M2 line (heat sink)          T3 (t) Lead block Cold mass shell P2 (W) R2 (T, t)R1 (T, t)HL (T1, t),T1 (t)HS (T2, t),T2(t)P1 (W)    4the cold mass shell (R2) and the lead block holding bracket to the M1 tube (R3). R1 is the ther-mal resistance of the stainless steel supports of the lead block. R1, R2 and R3 include the ther-mal contact resistances. P1 and P2 are respectively an optional power injected into the lead block (thermal or beam induced), and the power radiated to the shell by the 65 K cold mass thermal shield. R1, R2 and R3 are obtained over the whole temperature range (see Figure 6) during cool down from estimated enthalpy fluxes and measured temperatures at their bounda-ries [4]. The thermal model reproduced within a few percent the measured temperatures of the CC (see Figure 6), and confirmed sub-critical equilibrium temperatures of 6-7 K of the lead shielding blocks when nominal machine conditions will be applied.  R1-R2-R3  - range 13-300 K0.050.0100.0150.0200.0250.00 50 100 150 200 250 300 350Temp (K)K/WR3 mini pos 5R3 maxi pos 5R3 mini pos 1R3 maxi pos 1R1 mini pos 5R1 maxi pos 5R1 mini pos 1R1 maxi pos 1R2 mini Pos 5R3R1R2   Figure 6.  CC thermal resistances R1, R2 and R3 and model prediction of the test cool down   CONCLUSIONS  The thermal time constants and equilibrium temperatures of the shielding lead blocks and of the cold mass of the CC in nominal machine conditions, i.e. with the nominal radiated power from the thermal shield, are conform to the expected values.  A simplified thermal model of the connection cryostats based on three thermal resis-tances has been derived. The model allowed the determination of the lowest power that would have to be injected into the lead shielding in order to keep it from being superconducting. A power of 0.2 watts will be fed into the lead blocks via aluminum wires of 25 cm length and 2.5 mm in diameter, representing an additional extracted power per CC of 2 watts by the 1.9 K cooling system.   REFERENCES  1. Marque, S.  Connection Cryostat for LHC dispersion suppressors (DS), Functional Specification LHC-LE-ES-0001 (EDMS 113854), CERN, (2003). 2. Kozien, M., Marque, S.,  Thermo-Mechanical Analysis of the Updated Design Interconnection Cryostat, AT-CRI Technical Note 2004-07 (EDMS 474598), CERN, (2004). 3. Colombet, T., Résultats du test à froid réalisé sur le cryostat de connexion LE-C11-R5 durant le mois d'Août 2005  (EDMS 689776), CERN,  (2005). 4. Poncet, A., Cano, E.F., Colombet, T., Marque, S., Sitko, M., A simplified Thermal Simulation of the connec-tion cryostat. A solution to the problem of superconducting lead shielding, AT-CRI Technical Note 2005-08 (EDMS 689786), CERN, (2005). 5. Auchmann, B., Colombet, T., Devred, A., Koutchouk, J.P., Marque, S., Poncet, A., Possible resistive transi-tions in the Pb-Sb radiation shield in the connection cryostat and consequences on the beam, AT-MAS Technical Note 2006-05 (EDMS 754515), (2006).   0         50        100        150      200       250       300Time (hours) Thermal resistances R1, R2 and R3Measured values (K/Watt) between 13 K-300 KR3 R2  R1 200 1  1  50 Position 1 : cooldown of shell and leadcomparison of model to measurement0.88 Watts on shell Pos 10501001502002503000 50 100 150 200 250 300 350 400time (hours)Temperature (K)line M2lead pos 1-measuredshell pos 1-measuredlead -calculatedshell calculatedlead-calculated 50-5 Kshell -calculated 50-5 K       0         50       100      150       200      250      300      350      Time (hours) Temp (K) 300  250  200  150  100  50  Cool-down of ell and lead Comparison of model and measurementLine M2 (heat sink)Lead measured Lead model (dashed) Shell measured Shell model (dashed)R3 mini pos 5R3 maxi pos 5R3 mini pos 1R3 maxi pos 1R1 mini pos 5R1 maxi pos 5R1 mini pos 1R1 maxi pos 1R2 mini Pos 5K/W a b

Thermal Performance of the LHC Connection Cryostat

Abstract

Similar works

Full text

Available Versions

CERN Document Server